Sensitive hybridization techniques have been developed to selectively study the synthesis of avian tumor virus RNA in infected and transformed cells. Using these techniques, several size classes of virus-specific RNA can be isolated from both the nucleus and cytoplasm of transformed cells. Experiments currently in progress are attempting to determine 1) the molecular size of nuclear and cytoplasmic virus RNAs; 2) sequence composition of the various size classes of virus RNA; and 3) the biological activity of these RNAs in an in vitro protein synthesizing system. Experiments in progress are also aimed at the isolation of complementary DNA probes specific for a defined region of the RNA tumor virus genome. Synthesis of such probes will allow the detection and isolation of nuclear and cytoplasmic viral RNA species specific for a defined region of the genome.